Recent studies on the metabolism of glutathione indicate that glutathione is exported from many types of cells (see Meister, A. et al, Ann. Rev. Biochem. 52:711-760 (1983) and Larssen, A. et al, Ed., Functions of Glutathione - Biochemical, Physiological and Toxicological Aspects, Raven Press, New York (1983)). Glutathione exported from the liver accounts for most of the blood plasma level of glutathione, a large fraction of which is utilized by the kidney. However, this pathway of inter-organ transport of glutathione accounts for only some of the glutathione which is exported from cells. That is, export of glutathione from the kidneys into the blood and bile is several times greater than that from the liver. Such exported glutathione is utilized within the kidney tubule by the action of gamma-glutamyl transpeptidase and dipeptidase. Gamma-glutamyl transpeptidase characterizes the first step in the degradation of glutathione.
It has been shown that administration of certain inhibitors of gamma-glutamyl transpeptidase such as L-(or D-) gamma-glutamyl-(o-carboxy)phenylhydrazide, other hydrazides, 6-diazo-5-oxo-L-norleucine and L-(.alpha. S, 5S)-.alpha.-amino-3-chloro-4,5-dihydro-5-isoxazole acetic acid (hereinafter "AT-125") leads to substantial urinary excretion of glutathione (see Griffith, O. W. et al, Proc. Natl. Acad. Sci., USA, 76:268-272 (1979) and Griffith, O. W. et al, Proc. Natl. Acad. Sci., USA, 76:5606-5610 (1979)).
Although the isomers of gamma-glutamyl-(o-carboxy)phenylhydrazide are good inhibitors for gamma-glutamyl transpeptidase, they are split to a slight extent by gamma-glutamyl transpeptidase. This leads to the formation of o-carboxyphenylhydrazine, which is toxic and doses of more than 2 mM per kilogram of body weight have been found to be fatal in mice. Further, although AT-125 is a potent gamma-glutamyl transpeptidase inhibitor, it also inhibits a number of other enzymes, for example, glutamine amidotransferases (see Hanka, L. J. et al, Cancer Chemo. Rep. 57:141-148 (1973) and Neil, G. L. et al, Cancer Res. 39:852 (1979)).
Glutathione reacts with various endogenous and exogenous (including toxic compounds) to form S-substituted glutathione conjugates. These conjugates are usually metabolized. The first step of such metabolism is the cleavage of the gamma-glutamyl group from the glutathione conjugate by gamma-glutamyl transpeptidase. For example, methylchloride forms a conjugate with glutathione and metabolism of the resulting glutathione conjugate leads to formation of a toxic product, though to be H.sub.2 S. Inhibition of gamma-glutamyl transpeptidase by AT-125 inhibits conversion of the glutathione conjugate to the toxic product and thus decreases the toxicity of methyl chloride in mice (see White, R. D. et al, Pharm. 24:172 (1982)). However, since AT-125 is highly toxic and a non-specific inhibitor of gamma-glutamyl transpeptidase it can not be advantageously employed.
Moreover, methyl mercury forms a complex with glutathione and is excreted in this form in the urine. Excretion of this toxic compound as a glutathione complex is greatly increased after administration of AT-125 (see Gregus, Z. et al, The Toxicologist 6:150 (1986)). Again, since AT-125 is highly toxic and a non-specific inhibitor of gamma-glutamyl transpeptidase it can not be advantageously employed.
In addition, selenium poisoning is accompanied by incorporation of selenium in place of sulfur in glutathione. Heretofore, bromobenzene has been given to selenium-intoxicated animals on the premise that this would stimulate the urinary excretion of the selenium analog of glutathione. However, the use of bromobenzene is disadvantageous because, like AT-125, it is highly toxic and a non-specific inhibitor of gamma-glutamyl transpeptidase (see Moxon, A. L. et al, J. Biol. Chem. 132:785-786 (1940), Lemley, R. E., J. Lancet 60:258 (1940) and Westfall, B. B. et al, J. Pharmacol. 72:245-251 (1941)).
As discussed above, AT-125 and other inhibitors of gamma-glutamyl transpeptidase are toxic and non-specific inhibitors. Thus, these inhibitors are disadvantageous for combatting renal toxicity.
It has been found that gamma-glutamyl amino acids inhibit, in vivo, gamma-glutamyl transpeptidase and the administration of gamma-glutamyl amino acids leads to glutathionuria (see Anderson, M. E. et al, Fed. Proc. 41:5246 (1982), Meister, A. et al, Ann. Rev. Biochem. 52:711-760 (1983) and Anderson, M. E. et al, Proc. Natl. Acad. Sci., USA, 80:707-711 (1983)).
The present invention was developed in order to overcome the above-described disadvantages of known inhibitors of gamma-glutamyl transpeptidase in combatting renal toxicity and is based in part on the finding that in the present invention gamma-glutamyl amino acids are non-toxic and specific inhibitors of gamma-glutamyl transpeptidase and are useful for combatting renal toxicity due to metals or nephrotoxic drugs.
It is known that leukotrienes of the C and E type (type C being the initial glutathione conjugate normally found) are converted by gamma-glutamyl transpeptidase to leukotrienes of the D and F type, respectively (see Hammarstrom, S., Ann. Rev. Biochem. 52:355-377 (1983)). The C, D, E and F types of leukotrienes have different physiological effects, e.g., these leukotrienes can produce different allergic responses. Some are more active than others depending upon the type of system being studied. As a result, it is advantageous to increase formation of one type as compared to another. It has been found in the present invention that gamma-glutamyl amino acids are useful for this purpose.